The cannabis plant’s water and nutrient intake changes during vegetative growth. Transpiration is carried on at a rapid rate, requiring more water and nutrients. Higher levels of nitrogen are needed along with a balanced supply of potassium, phosphorous, calcium, magnesium, sulfur, and trace elements, which get used at an increased rate. The larger the plant becomes, the bigger the root system, and the faster the soil will dry out. The key to strong vegetative growth is supplying the roots with the perfect environment for strong growth and one that will discourage pests like fungus gnats that can cause a lot of damage to the fine root hairs that are essential for transferring water and nutrients from the soil up into the plant.
Vegetative growth is maintained with 16 or more hours of light. Since cannabis is photoperiod reactive, vegetative growth and flowering can be controlled by managing how many hours of light and dark the plants receive. This allows indoor horticulturists to control exactly when to initiate a flowering cycle.
Cannabis is a dioecious plant, being either male or female. Sometimes, hermaphrodite plants are produced as well, having both male and female flowers. When it comes to cannabis, the female plant is the most desirable as it is the one that produces the dense flower clusters used for ingestion. Male plants are only cultivated when seed production is the primary goal. Hermaphrodites are undesirable and should be removed from the garden as soon as they have been identified.
After about 4 weeks of vegetative growth, cannabis plants will produce pre-flowers that will reveal their sex. This is very useful since you will want to remove and destroy male plants that are taking up valuable real estate in the grow room and if your goal is to produce seed free sinsemilia buds. Cannabis plants grown under an 18 hour light regimen usually show pre-flowers before those that are grown under a continuous 24 hour light cycle.
When cannabis plants enter the flowering cycle, their chemistry changes and their nutrient requirements also change. Flowering plants need less nitrogen and increased amount of phosphorous and potassium to promote flower formation. Growth patterns change as well during the flowering cycle, chlorophyll production slows and branches and stems elongate.
Simply by changing the photoperiod to 12 hours of light and 12 hours of dark will initiate the flowering stage and within 2 weeks the first flowers will start to appear. If you provide adequate amounts of the right nutrients throughout the flowering stage, plants will continue to produce flower clusters for an average of 8 to 10 weeks. Some varieties of cannabis have longer flowering cycles and some will autoflower regardless of changing of the photoperiod. When purchasing seeds, you will be able to select from varieties of differing flowering stage durations, autoflowering varieties, feminized strains that will not produce any male plants, and of course THC and CBD percentages.
Selecting a variety that has been feminized will save you the time and space of producing the unwanted male plants that ultimately get discarded. And you only need to use a few of your seeds since they will all turn out female, and then you can save the rest for a later time. After your plants are about 2 months old, you can start taking cuttings from the most vigorous plants lower branches to make the clones with which you can fill the grow room.